Plasma arc torches are widely used for cutting metallic materials. A plasma arc torch generally includes a torch body, an electrode mounted within the body, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, a nozzle with a central exit orifice, electrical connections, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. A shield may also be employed to provide a shield gas flow to the area proximate the plasma arc. Gases used in the torch can be non-reactive (e.g. argon or nitrogen), or reactive (e.g. oxygen or air).
In operation, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). The pilot arc ionizes gas passing through the nozzle exit orifice. As the ionized gas reduces the electrical resistance between the electrode and the workpiece, the arc transfers from the nozzle to the workpiece. The torch may be operated in this transferred plasma arc mode, which is characterized by the conductive flow of ionized gas from the electrode to the workpiece, for the cutting of the workpiece.
Generally, there are two widely used techniques for generating a pilot plasma arc. One technique uses a high frequency, high voltage ("HFHV") signal coupled to a DC power supply and the torch. The HFHV signal is typically provided by a generator associated with the power supply. The HFHV signal induces a spark discharge in the plasma gas flowing between the electrode and the nozzle, and this discharge provides a current path. The pilot arc is formed between the electrode and the nozzle with the voltage existing across them.
The other technique for generating a pilot plasma arc is known as contact starting. Contact starting is advantageous because it does not require high frequency equipment and, therefore, is less expensive and does not generate electromagnetic interference. In one form of contact starting, the electrode is manually placed into physical and electrical contact with the workpiece. A current is then passed through the electrode to the workpiece and the arc is struck by manually retracting the electrode from the workpiece. Other contact starting systems may employ a translatable electrode in combination with a fixed nozzle, such as that disclosed in U.S. Pat. No. 4,791,268, which is assigned to the same assignee as the instant invention.
During operation of a plasma arc torch, a significant temperature rise occurs in the electrode. In systems which employ a movable electrode, passive conductive cooling of the electrode by adjacent structure is reduced due to the need to maintain sliding fit clearances therebetween. Such clearances reduce heat transfer efficiencies relative to fixed electrode designs employing threaded connections or interference fits. Accordingly, active cooling arrangements have been developed such as those disclosed in U.S. Pat. No. 4,902,871 ("the '871 patent"), which is assigned to the same assignee as the present invention. Briefly, the '871 patent describes an electrode having a spiral gas flow passage circumscribing an enlarged shoulder portion thereof. Enhanced heat transfer and extended electrode life are realized due to the increased surface area of the electrode exposed to the cool, accelerated gas flow.